mt76: mt76x02: omit beacon slot clearing

mt76 hw does not send beacons from beacon slots, if the corresponding
bitmask is set accordingly. Therefore we can omit clearing the beacon
memory. Clearing uses many usb calls, if usb drivers are used. These
calls unnecessarily slow down the beacon tasklet. Thanks to Stanislaw
Gruzska for pointing this out.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>